The present invention relates to a Class D audio power amplifier and more specifically to a method for minimizing audible click noise from a speaker upon shutdown or startup.
FIG. 1 shows an example of a Class D audio amplifier including a switching stage of two switches 4 and 5 series connected at a node VS. Gates of the switches 4 and 5 are controlled by a controller 7 that receives PWM signals for driving and shutdown/startup signals 1 to start and stop the switching stage. The circuit further comprises an LC filter that includes an inductor 2 connected between the VS node and a capacitor 3. As illustrated in FIG. 1, a speaker cut-off switch 6 connects a speaker to the node between the inductor 2 and the capacitor 3.
In the Class D amplifiers, during transition intervals from a total shutdown state to a normal operation state at startup and from the normal operation state to the total shutdown state at shutdown, the energy stored within the output demodulation filter stage 2 and 3 of the amplifier is transitioning to or from a zero energy state. These energy transfers and/or dissipation stages cause a transient voltage excursion on the speaker that contains audible frequency components. Hence click noise is heard from the speaker. The energy that is transferred to the speaker is dependent on both, the current in the output filter inductor 2 and the voltage across the output capacitor 3. Since this noise is an unwanted feature, a bulky speaker cutoff relay 6 is commonly inserted between the amplifier and the speaker to disconnect the speaker during times when these transients are active.
At startup there is a transient, from the initial no energy shutdown state to the normal operation state. The transient is stored within the output filter stage, where a certain amount of energy is circulating between the output stage inductor 2 and capacitor 3. To transition between these two states, energy is transferred to the output filter. This transient causes a rapid change in the voltage across the output filter capacitor and therefore the speaker, which contains audible frequency components.
At shutdown there is a transient from the normal operation state to the shutdown state. A certain amount of energy, circulating between the output stage inductor and capacitor having no energy, is stored within the output filter stage. To transition between these two states, energy within the output filter has to be recovered and/or be dissipated. This transient causes a rapid change of the voltage across the output filter capacitor 3 and therefore the speaker, which contains audible frequency components.
As illustrated in FIG. 2a, during the OFF period t0, both switches 4 and 5 are OFF, resulting in a high impedance state at the node VS. At transition over to the normal operation state at period t2, the PWM driven switching at the node VS is started, and depending on the initial timing, the audible transient response, illustrated by speaker voltage, can be quite significant. Because of this transient, the speaker is only connected to the output after the transient has passed using the switch or relay 6. FIG. 2b is an enlargement of the graphs of FIG. 2a, it illustrates a steady increase in the speaker voltage during the first four switching cycles.
Similarly at shutdown, the speaker is disconnected prior to the shutdown voltage transient is dissipated. As illustrated in FIG. 3a, depending on the final switching timing, the audible transient response can also be quite significant. With the switching ending at maximum inductor energy and current, the audible transient speaker voltage will be maximum. FIG. 3b is an enlargement of the graphs of FIG. 3a, it illustrates the last two switching cycles and speaker voltage at period t2.
What is needed is a click noise-less startup and shutdown method that eliminates a need of a speaker cut-off switch, saving system cost and space.